1. Field of the Invention
The present invention relates to a color processing apparatus configured to perform color correction according to color stabilization accuracy of an image output device.
2. Description of the Related Art
In performing color matching between different devices (e.g., a monitor and a printer), a conventional system uses the following formula (1) to obtain XYZ tri-stimulus values of illuminant colors (e.g., monitor colors) and uses the following formula (2) to obtain XYZ tri-stimulus values of object colors (e.g., printed product colors). Furthermore, the conventional system uses the following formula (3) to obtain CIELab values (hereinafter referred to as “Lab values”) from the XYZ tri-stimulus values, as color values in a device-independent color space. Then, the system performs color reproduction such that an input image matches an output image in Lab values.X=k∫visP(λ)· x(λ)·dλY=k∫visP(λ)· y(λ)·dλZ=k∫visP(λ)· z(λ)·dλk=683.0  (1)In formula (1), P(λ) represents a spectral radiance of illuminant color, and x(λ), y(λ), and z(λ) represent color matching functions.
                                                        X              =                            ⁢                              k                ⁢                                                      ∫                    vis                                    ⁢                                                            R                      ⁡                                              (                        λ                        )                                                              ·                                          P                      ⁡                                              (                        λ                        )                                                              ·                                                                  x                        _                                            ⁡                                              (                        λ                        )                                                              ·                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Y              =                            ⁢                              k                ⁢                                                      ∫                    vis                                    ⁢                                                            R                      ⁡                                              (                        λ                        )                                                              ·                                          P                      ⁡                                              (                        λ                        )                                                              ·                                                                  y                        _                                            ⁡                                              (                        λ                        )                                                              ·                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Z              =                            ⁢                              k                ⁢                                                      ∫                    vis                                    ⁢                                                            R                      ⁡                                              (                        λ                        )                                                              ·                                          P                      ⁡                                              (                        λ                        )                                                              ·                                                                  z                        _                                            ⁡                                              (                        λ                        )                                                              ·                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              k              =                            ⁢                              100                                                      ∫                    vis                                    ⁢                                                            P                      ⁡                                              (                        λ                        )                                                              ·                                                                  y                        _                                            ⁡                                              (                        λ                        )                                                              ·                                                                                  ⁢                                          ⅆ                      λ                                                                                                                              (        2        )            In formula (2), R(λ) represents a spectral reflectance of object color, P(λ) represents a spectral radiance of illuminant color, and x(λ), y(λ), and z(λ) represent color matching functions.
                                                                        L                *                            =                            ⁢                                                116                  *                                      f                    ⁡                                          (                                              Y                                                  Y                          n                                                                    )                                                                      -                16                                                                                                        a                *                            =                            ⁢                              500                *                                  {                                                            f                      ⁡                                              (                                                  X                                                      X                            n                                                                          )                                                              -                                          f                      ⁡                                              (                                                  Y                                                      Y                            n                                                                          )                                                                              }                                                                                                                        b                *                            =                            ⁢                              200                *                                  {                                                            f                      ⁡                                              (                                                  Y                                                      Y                            n                                                                          )                                                              -                                          f                      ⁡                                              (                                                  Z                                                      Z                            n                                                                          )                                                                              }                                                                                        (        3        )            In formula (3), Xn, Yn, and Zn represent X, Y, and Z values of a white color point.
      f    ⁡          (              X                  X          n                    )        =      {        ⁢                                                                                      ⁢                                                                            (                                              X                                                  X                          n                                                                    )                                                              1                      3                                                        ,                                                            when                      ⁢                                                                                          ⁢                                              X                                                  X                          n                                                                                      >                    0.008856                                                                                                                                                                7.787                    ⁢                                          (                                              X                                                  X                          n                                                                    )                                                        +                                      16                    116                                                  ,                                                      when                    ⁢                                                                                  ⁢                                          X                                              X                        n                                                                              ≤                  0.008856                                                                    ⁢                                  ⁢                  f          ⁡                      (                          Y                              Y                n                                      )                              =              {                ⁢                                                                                                                      ⁢                                                                                            (                                                      Y                                                          Y                              n                                                                                )                                                                          1                          3                                                                    ,                                                                        when                          ⁢                                                                                                          ⁢                                                      Y                                                          Y                              n                                                                                                      >                        0.008856                                                                                                                                                                                                            7.787                        ⁢                                                  (                                                      Y                                                          Y                              n                                                                                )                                                                    +                                              16                        116                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Y                                                      Y                            n                                                                                              ≤                      0.008856                                                                                            ⁢                                                  ⁢                          f              ⁡                              (                                  Z                                      Z                    n                                                  )                                              =                      {                        ⁢                                                                                                    ⁢                                                                                            (                                                      Z                                                          Z                              n                                                                                )                                                                          1                          3                                                                    ,                                                                        when                          ⁢                                                                                                          ⁢                                                      Z                                                          Z                              n                                                                                                      >                        0.008856                                                                                                                                                                                                            7.787                        ⁢                                                  (                                                      Z                                                          Z                              n                                                                                )                                                                    +                                              16                        116                                                              ,                                                                  when                        ⁢                                                                                                  ⁢                                                  Z                                                      Z                            n                                                                                              ≤                      0.008856                                                                                                              
However, image display/output devices have respective particular color reproduction ranges (i.e., different color gamuts). Thus, some reproducible colors in a specific device may not be reproduced by another device.
Hence, a conventional color matching system performs color gamut mapping processing that can replace the colors that cannot be reproduced by an object device (i.e., out-of-gamut colors) with reproducible colors (i.e., in-gamut colors).
As discussed in Japanese Patent Application Laid-Open No. 2003-283846, human eyes are sensitive to a change in an achromatic color compared to a change in a chromatic color. Thus, a general color gamut mapping system performs processing for greatly compressing the color saturation in a chromatic color region (i.e., high color saturation region) while maintaining the color saturation in an achromatic color region (i.e., low color saturation region).
As discussed in Japanese Patent Application Laid-Open No. 2005-318478, image output devices can reproduce colors of an image using various physical characteristics. However, the reproduced colors tend to fluctuate due to environmental changes (e.g., temperature, moisture, etc.). Calibration technique enables correcting color fluctuations (color deviations), if caused by environmental changes, with reference to target characteristics determined in the color design of a device. Thus, the calibration technique can realize stable color reproduction.
However, even after the color stabilization accuracy is improved with the calibration technique, errors may arise due to the difficulty in completely realizing the target characteristics. Furthermore, fluctuations in an image output device may occur in both chromatic colors and achromatic colors. If an output color fluctuates in a low color saturation region, human eyes may recognize a reproduced color having a color saturation higher than the target color saturation as color fluctuation. Human eyes are sensitive to a fluctuation in the low color saturation region compared to that in the high color saturation region.